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Physics - Friction

Friction

Diagram of frictional forces acting on a car's wheel

Friction is a force that acts between an object and the surface it is
moving along (or could move along). It always opposes motion at the point
of contact. For an object being dragged along, friction is in the opposite
direction to the force being used to drag the object.

For example, in the diagram below, there are two horizontal forces
acting,_ F_ to the right and to the left:

Friction can also be used to propel an object forward, by using a driven
wheel. This is what happens in a car. In this case, the wheel is rotated
(by the engine). In rotating, the bottom of the wheel attempts to slide
past the road. If there is sufficient friction, the wheel won't slide and
the whole wheel, and therefore the car, moves forward. Note, however, that
friction is still opposing the motion at the point of application. This
sort of friction is often called a driving force. If it is the only
horizontal force acting on the object, the car accelerates in the direction
of the frictional force.

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